The EGF receptor is broadly distributed on cells of epithelial and epidermal tissues. Upon stimulation by EGF, the receptor transduces a potent mitogenic signal. Thus, the EGF-EGF R tissues such as skin, colon, respiratory epithelia and liver. Studies of the physiology of the EGF signalling system have been complicated due to the discovery of several new EGF-like proteins which may act by binding to the EGF R. We have investigated the structure and synthesis of the EGF R in quiescent, growing and transformed liver cells. We have noted substantial changes in EGF R synthesis in these various states and have uncovered a truncated form of the EGF R from liver. The following three objectives stem from our prior work. I. To understand the regulation of EGF R synthesis. EGF increases EGF receptor synthesis in quiescent hepatic epithelial cells. Paradoxically, cycling hepatic cells decrease their EGF R synthesis. Epithelial cancers, the majority of human malignancies, can exhibit either extraordinary overexpression of EGF R mRNA or total abolition of EGF R synthesis. Aim 1 details studies of transcriptional control of rat EGF R synthesis in a series of experimental models that exhibit positive or negative regulation. II. To understand the regulation and function of a truncated form of the EGF R. We have isolated a cDNA derived from a 2.7 kB EGF receptor mRNA in rat liver. This cDNA encodes an EGF R extracellular domain that is truncated so that it is secreted, yet is similar enough in structure to the normal EGF receptor that it retains the ability to bind 125I-EGF. Aim 2 will test whether this protein modulates EGF receptor signalling by binding EGF-like peptides or by interacting with full length receptors. These studies will use expression vectors, purified protein and transgenic mice to elucidate this protein's function. III. To determine whether the EGF R is involved in liver regeneration. EGF stimulates the growth of hepatocyte and other liver cells. TGFalpha-bearing transgenic mice have extremely hyperplastic livers. Aim 3 will use in situ hybridization to study the localized expression of EGF R and EGF-like peptides and to correlate expression with specific cell types or localized patterns of DNA synthesis. In Aim 4, we will make anti-rat EGF R monoclonals and select MoAbs that block EGF and TGFalpha binding to the EGF R. Antibodies will then be tested for their ability to block ligand- directed EGF R signalling and mitogenesis in vitro and then in vivo. MoAbs that meet these criteria will be useful in understanding the role of the EGF R in physiologic growth processes such as liver regeneration.